Hindawi Publishing Corporation Scientifica Volume 2013, Article ID 620504, 14 pages http://dx.doi.org/10.1155/2013/620504
Review Article The Role of the Intraplaque Vitamin D System in Atherogenesis Federico Carbone1,2 and Fabrizio Montecucco1,3 1
Department of Internal Medicine, University of Genoa School of Medicine, IRCCS Azienda Ospedaliera Universitaria San Martino-IST Istituto Nazionale per la Ricerca sul Cancro, 6 Viale Benedetto XV, 16132 Genoa, Italy 2 Cardiology Division, Foundation for Medical Researches, Department of Internal Medicine, University of Geneva, 64, Avenue de la Roseraie, 1211 Geneva, Switzerland 3 Division of Laboratory Medicine, Department of Genetics and Laboratory Medicine, Geneva University Hospitals, 4 rue Gabrielle-Perret-Gentil, 1205 Geneva, Switzerland Correspondence should be addressed to Federico Carbone; [email protected] Received 18 November 2013; Accepted 10 December 2013 Academic Editors: E. P. Cherniack, T. Minamino, K. Paraskevas, and R. Van Dam Copyright © 2013 F. Carbone and F. Montecucco. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Vitamin D has been shown to play critical activities in several physiological pathways not involving the calcium/phosphorus homeostasis. The ubiquitous distribution of the vitamin D receptor that is expressed in a variety of human and mouse tissues has strongly supported research on these “nonclassical” activities of vitamin D. On the other hand, the recent discovery of the expression also for vitamin D-related enzymes (such as 25-hydroxyvitamin D-1𝛼-hydroxylase and the catabolic enzyme 1,25-dihydroxyvitamin D-24-hydroxylase) in several tissues suggested that the vitamin D system is more complex than previously shown and it may act within tissues through autocrine and paracrine pathways. This updated model of vitamin D axis within peripheral tissues has been particularly investigated in atherosclerotic pathophysiology. This review aims at updating the role of the local vitamin D within atherosclerotic plaques, providing an overview of both intracellular mechanisms and cell-to-cell interactions. In addition, clinical findings about the potential causal relationship between vitamin D deficiency and atherogenesis will be analysed and discussed.
1. Introduction Since its discovery in the early 1900s, the role of vitamin D has been limited to calcium/phosphate homeostasis through a predominant action on the kidney, intestine, and bone [1]. On the contrary, evidence in recent decades has suggested that vitamin D might play a critical role in many other metabolic pathways, referred as “nonclassical effects” [2]. Thus, vitamin D is currently under investigation in cancer [3], autoimmune disorders [4], infections [5], and neurological [6] and cardiovascular (CV) diseases. A large amount of observational studies has shown that vitamin D deficiency is associated with a wide range of CV risk factors [7], as well as poor CV outcome [8], but more recent findings from interventional trial have weakened this initial enthusiasm with a more sceptical view. Ultimately, Brandenburg correctly stated: “there should be less persuasive observational associative data, but more convincing interventional results in the field of vitamin D” [9]. Certainly, a critical analysis of literature has revealed several
limitations especially in study design, but also the newer insights about the local activity of vitamin D within peripheral tissues might explain the conflicting results between interventional and observational studies. In this new research approach, 25-hydroxyvitamin D-1𝛼-hydroxylase (CYP27B1) is emerging as a main regulator of the extrarenal vitamin D system along with the catabolic enzyme 1,25-dihydroxyvitamin D-24-hydroxylase (CYP24A1) and vitamin D receptor (VDR). The aim of this review is to update the current evidence about the role of vitamin D in the pathophysiology of atherosclerosis and suggest a critical basis for future investigations.
2. Vitamin D Signalling The availability of vitamin D is largely dependent on sunlight exposure (more than 80% of the requirements). In skin, ultraviolet-B (UVB) radiation induces the conversion of 7dehydrocholesterol to the inactive precursor of vitamin D,
2 through a photosynthetic reaction which evolved over 750 million years ago [10]. Subsequently, the 25-hydroxylation in the liver generates the 25-hydroxyvitamin D [25(OH) vitamin D or calcidiol] [11], which is biologically inactive but nonetheless used as marker of vitamin D status because of being stable, largely circulating, and easy to quantify. Calcidiol becomes active after conversion to 1,25-dihydroxyvitamin D [1,25(OH)2 vitamin D or calcitriol] which occurs through the action of CYP27B1, the rate-limiting enzyme [12]. Accordingly, CYP27B1 activity is tightly regulated with feedback control mechanisms (at least in the kidney) involving the parathyroid hormone (PTH), calcitonin, 1,25(OH)2 vitamin D itself [13], and CYP24A1 (the catabolic enzyme of vitamin D) [14]. The biological response to 1,25(OH)2 vitamin D is mediated by VDR, a DNA-binding transcription factor member of the nuclear receptor superfamily. VDR activation requires the binding to both 1,25(OH)2 vitamin D and one of retinoid X receptors (RXR 𝛼, 𝛽, or 𝛾). Only in this heterodimeric form VDR complex recognizes the vitamin D response elements (VDRE), repeated sequences of 6 hexamers in the promoter region of target gene. Furthermore, since VDR may regulate 3% to 5% of human genome, allosteric influences, VDRE location, and epigenetic modification of DNA and histones modulate the VDR activity in the different cell types [15]. An additional feature shown by VDR (and by the whole nuclear receptor superfamily) is the ability to bind multiple lipophilic ligands, thus amplifying the vitamin D signalling activity. Interestingly, an extranuclear expression of VDR (on cell surface membrane and mitochondria) was recently discovered [16, 17] and shown to trigger nongenomic rapid responses [18]. Unlike the genomic responses (generally taking several hours till days to be fully manifest), these rapid nongenomic responses are generated in a shorter period of time (1-2 to 45 minutes). As already recognized for other steroidal hormones [19–21], plasma membrane caveolae are involved in vitamin D-induced rapid responses. Caveolae are localized within the lipid-rafts (microdomains of the plasma membrane enriched in sphingolipids and cholesterol) and might promote intracellular responses by flask-shaped membrane invagination [22]. VDR was found to be closely localized to caveolae [23], as also suggested by functional studies [24]. The VDR-caveolae complex may activate several downstream intracellular signalling cascades involving kinases, phosphatases, and ion channels as well as modulate gene expression, in a cross-talk with the classical genomic effects of vitamin D [25]. Ultimately, these recent insights, together with the ability to bind multiple lipophilic ligands (feature shared by the whole nuclear receptor superfamily), further increased complexity in vitamin D signalling pathways.
3. Vitamin D System and Atherosclerosis: Clinical Findings Acute ischemic atherosclerotic complications are the leading cause of mortality and morbidity worldwide [26]. To date, it is commonly accepted that atherosclerotic plaque development is orchestrated by chronic low-grade inflammatory processes
Scientifica occurring within the arterial wall, in peripheral organs, and in the systemic circulation [27]. Endothelial dysfunction is a very early step in atherogenesis, especially at sites characterized by disturbed laminar flow. This pathophysiological event promotes subendothelial accumulation of low density lipoproteins (LDLs) [28]. Within the subintimal space of the arterial wall, LDLs (whether in native form or modified by oxidative stress) trigger inflammatory and vascular resident cells to produce several mediators attracting circulating leukocytes, including monocytes [29], neutrophils [30], and lymphocytes [31]. This chronic inflammatory process is responsible for the atherosclerotic plaque structure (including the necrotic lipid core and the fibrous cap) and promotes plaque instability [32]. Several observational studies and recent meta-analyses in humans showed that circulating 25(OH) vitamin D was inversely correlated with poor CV outcomes [8, 33, 34]. However, the first randomized clinical trials have provided even more discouraging results [34, 35]. In addition, also studies investigating the potential relationship between serum vitamin D and atherosclerotic plaque vulnerability have provided ambiguous results. For instance, studies focusing on carotid intima-media thickness (cIMT), a well-recognized biomarker of subclinical atherosclerosis also associated with a wide range of CV risk factors and CV diseases [36], showed a potential relationship between vitamin D deficiency and atherogenesis (Table 1). In particular, Deleskog and coworkers, in a longitudinal evaluation of 3,430 patients at high cardiovascular risk but without prevalent disease, failed to show an increased cIMT progression in vitamin D deficient patients when compared with the group with sufficient vitamin D [37]. On the other hand, the significant association between low vitamin D levels and a wide range of CV risk factor observed in this cohort did not prove any potential connections between vitamin D and clinical atherosclerotic outcomes. These recent findings are in accordance with previous observational studies. The research groups of Targher et al. and Liu et al. demonstrated an inverse correlation between vitamin D levels and cIMT severity [38, 39]. Among a subgroup of patients with end-stage renal disease, only Kra´sniak and colleagues [40] showed a linear inverse correlation between 25(OH) vitamin D and cIMT. On the other hand, the case-control study of Briese et al. [41] and the crosssectional analysis of Zang and coworkers [42] failed to prove any association. Likewise, in two observational cohorts of HIV-infected patients, vitamin D deficiency was showed as correlated with cIMT severity [43, 44]. However, these results were not confirmed by a recent larger simple size crosssectional study [45]. Furthermore, recent studies (enrolling community-dwelling healthy subjects) failed to prove any relationships between vitamin D deficiency and cIMT. However, although these studies enrolled a large cohort of patients, they were designed with serious limitations. For instance, both geographical and seasonal differences in sunlight exposure might influence vitamin D status evaluation, as well as African race and old age. In addition, large simple size studies of vitamin D have been shown to underestimate other confounding factors, including differences in physical activity and dietary habits of patients, which may have significantly impacted the results [46–53]. As reported in Table 2, another
Scientifica
3
Table 1: Observational studies investigating the relationship between vitamin D and carotid intima-media thickness.
Author
Year
Briese et al. 2006 [41]
Study design (sample size)
Country (ethnicity) Age
Correlation (lower range of 25(OH)D)
Findings
Case-control (40 ESRD patients and 40 matched healthy controls)
Germany (Caucasian) Mean 23.6 years
No (linear correlation)
There was no difference in CCA-IMT between the two groups. This study failed to correlate 25(OH)D and cIMT.
Italy (Caucasian) 50–65 years
Yes (
Review Article The Role of the Intraplaque Vitamin D System in Atherogenesis Federico Carbone1,2 and Fabrizio Montecucco1,3 1
Department of Internal Medicine, University of Genoa School of Medicine, IRCCS Azienda Ospedaliera Universitaria San Martino-IST Istituto Nazionale per la Ricerca sul Cancro, 6 Viale Benedetto XV, 16132 Genoa, Italy 2 Cardiology Division, Foundation for Medical Researches, Department of Internal Medicine, University of Geneva, 64, Avenue de la Roseraie, 1211 Geneva, Switzerland 3 Division of Laboratory Medicine, Department of Genetics and Laboratory Medicine, Geneva University Hospitals, 4 rue Gabrielle-Perret-Gentil, 1205 Geneva, Switzerland Correspondence should be addressed to Federico Carbone; [email protected] Received 18 November 2013; Accepted 10 December 2013 Academic Editors: E. P. Cherniack, T. Minamino, K. Paraskevas, and R. Van Dam Copyright © 2013 F. Carbone and F. Montecucco. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Vitamin D has been shown to play critical activities in several physiological pathways not involving the calcium/phosphorus homeostasis. The ubiquitous distribution of the vitamin D receptor that is expressed in a variety of human and mouse tissues has strongly supported research on these “nonclassical” activities of vitamin D. On the other hand, the recent discovery of the expression also for vitamin D-related enzymes (such as 25-hydroxyvitamin D-1𝛼-hydroxylase and the catabolic enzyme 1,25-dihydroxyvitamin D-24-hydroxylase) in several tissues suggested that the vitamin D system is more complex than previously shown and it may act within tissues through autocrine and paracrine pathways. This updated model of vitamin D axis within peripheral tissues has been particularly investigated in atherosclerotic pathophysiology. This review aims at updating the role of the local vitamin D within atherosclerotic plaques, providing an overview of both intracellular mechanisms and cell-to-cell interactions. In addition, clinical findings about the potential causal relationship between vitamin D deficiency and atherogenesis will be analysed and discussed.
1. Introduction Since its discovery in the early 1900s, the role of vitamin D has been limited to calcium/phosphate homeostasis through a predominant action on the kidney, intestine, and bone [1]. On the contrary, evidence in recent decades has suggested that vitamin D might play a critical role in many other metabolic pathways, referred as “nonclassical effects” [2]. Thus, vitamin D is currently under investigation in cancer [3], autoimmune disorders [4], infections [5], and neurological [6] and cardiovascular (CV) diseases. A large amount of observational studies has shown that vitamin D deficiency is associated with a wide range of CV risk factors [7], as well as poor CV outcome [8], but more recent findings from interventional trial have weakened this initial enthusiasm with a more sceptical view. Ultimately, Brandenburg correctly stated: “there should be less persuasive observational associative data, but more convincing interventional results in the field of vitamin D” [9]. Certainly, a critical analysis of literature has revealed several
limitations especially in study design, but also the newer insights about the local activity of vitamin D within peripheral tissues might explain the conflicting results between interventional and observational studies. In this new research approach, 25-hydroxyvitamin D-1𝛼-hydroxylase (CYP27B1) is emerging as a main regulator of the extrarenal vitamin D system along with the catabolic enzyme 1,25-dihydroxyvitamin D-24-hydroxylase (CYP24A1) and vitamin D receptor (VDR). The aim of this review is to update the current evidence about the role of vitamin D in the pathophysiology of atherosclerosis and suggest a critical basis for future investigations.
2. Vitamin D Signalling The availability of vitamin D is largely dependent on sunlight exposure (more than 80% of the requirements). In skin, ultraviolet-B (UVB) radiation induces the conversion of 7dehydrocholesterol to the inactive precursor of vitamin D,
2 through a photosynthetic reaction which evolved over 750 million years ago [10]. Subsequently, the 25-hydroxylation in the liver generates the 25-hydroxyvitamin D [25(OH) vitamin D or calcidiol] [11], which is biologically inactive but nonetheless used as marker of vitamin D status because of being stable, largely circulating, and easy to quantify. Calcidiol becomes active after conversion to 1,25-dihydroxyvitamin D [1,25(OH)2 vitamin D or calcitriol] which occurs through the action of CYP27B1, the rate-limiting enzyme [12]. Accordingly, CYP27B1 activity is tightly regulated with feedback control mechanisms (at least in the kidney) involving the parathyroid hormone (PTH), calcitonin, 1,25(OH)2 vitamin D itself [13], and CYP24A1 (the catabolic enzyme of vitamin D) [14]. The biological response to 1,25(OH)2 vitamin D is mediated by VDR, a DNA-binding transcription factor member of the nuclear receptor superfamily. VDR activation requires the binding to both 1,25(OH)2 vitamin D and one of retinoid X receptors (RXR 𝛼, 𝛽, or 𝛾). Only in this heterodimeric form VDR complex recognizes the vitamin D response elements (VDRE), repeated sequences of 6 hexamers in the promoter region of target gene. Furthermore, since VDR may regulate 3% to 5% of human genome, allosteric influences, VDRE location, and epigenetic modification of DNA and histones modulate the VDR activity in the different cell types [15]. An additional feature shown by VDR (and by the whole nuclear receptor superfamily) is the ability to bind multiple lipophilic ligands, thus amplifying the vitamin D signalling activity. Interestingly, an extranuclear expression of VDR (on cell surface membrane and mitochondria) was recently discovered [16, 17] and shown to trigger nongenomic rapid responses [18]. Unlike the genomic responses (generally taking several hours till days to be fully manifest), these rapid nongenomic responses are generated in a shorter period of time (1-2 to 45 minutes). As already recognized for other steroidal hormones [19–21], plasma membrane caveolae are involved in vitamin D-induced rapid responses. Caveolae are localized within the lipid-rafts (microdomains of the plasma membrane enriched in sphingolipids and cholesterol) and might promote intracellular responses by flask-shaped membrane invagination [22]. VDR was found to be closely localized to caveolae [23], as also suggested by functional studies [24]. The VDR-caveolae complex may activate several downstream intracellular signalling cascades involving kinases, phosphatases, and ion channels as well as modulate gene expression, in a cross-talk with the classical genomic effects of vitamin D [25]. Ultimately, these recent insights, together with the ability to bind multiple lipophilic ligands (feature shared by the whole nuclear receptor superfamily), further increased complexity in vitamin D signalling pathways.
3. Vitamin D System and Atherosclerosis: Clinical Findings Acute ischemic atherosclerotic complications are the leading cause of mortality and morbidity worldwide [26]. To date, it is commonly accepted that atherosclerotic plaque development is orchestrated by chronic low-grade inflammatory processes
Scientifica occurring within the arterial wall, in peripheral organs, and in the systemic circulation [27]. Endothelial dysfunction is a very early step in atherogenesis, especially at sites characterized by disturbed laminar flow. This pathophysiological event promotes subendothelial accumulation of low density lipoproteins (LDLs) [28]. Within the subintimal space of the arterial wall, LDLs (whether in native form or modified by oxidative stress) trigger inflammatory and vascular resident cells to produce several mediators attracting circulating leukocytes, including monocytes [29], neutrophils [30], and lymphocytes [31]. This chronic inflammatory process is responsible for the atherosclerotic plaque structure (including the necrotic lipid core and the fibrous cap) and promotes plaque instability [32]. Several observational studies and recent meta-analyses in humans showed that circulating 25(OH) vitamin D was inversely correlated with poor CV outcomes [8, 33, 34]. However, the first randomized clinical trials have provided even more discouraging results [34, 35]. In addition, also studies investigating the potential relationship between serum vitamin D and atherosclerotic plaque vulnerability have provided ambiguous results. For instance, studies focusing on carotid intima-media thickness (cIMT), a well-recognized biomarker of subclinical atherosclerosis also associated with a wide range of CV risk factors and CV diseases [36], showed a potential relationship between vitamin D deficiency and atherogenesis (Table 1). In particular, Deleskog and coworkers, in a longitudinal evaluation of 3,430 patients at high cardiovascular risk but without prevalent disease, failed to show an increased cIMT progression in vitamin D deficient patients when compared with the group with sufficient vitamin D [37]. On the other hand, the significant association between low vitamin D levels and a wide range of CV risk factor observed in this cohort did not prove any potential connections between vitamin D and clinical atherosclerotic outcomes. These recent findings are in accordance with previous observational studies. The research groups of Targher et al. and Liu et al. demonstrated an inverse correlation between vitamin D levels and cIMT severity [38, 39]. Among a subgroup of patients with end-stage renal disease, only Kra´sniak and colleagues [40] showed a linear inverse correlation between 25(OH) vitamin D and cIMT. On the other hand, the case-control study of Briese et al. [41] and the crosssectional analysis of Zang and coworkers [42] failed to prove any association. Likewise, in two observational cohorts of HIV-infected patients, vitamin D deficiency was showed as correlated with cIMT severity [43, 44]. However, these results were not confirmed by a recent larger simple size crosssectional study [45]. Furthermore, recent studies (enrolling community-dwelling healthy subjects) failed to prove any relationships between vitamin D deficiency and cIMT. However, although these studies enrolled a large cohort of patients, they were designed with serious limitations. For instance, both geographical and seasonal differences in sunlight exposure might influence vitamin D status evaluation, as well as African race and old age. In addition, large simple size studies of vitamin D have been shown to underestimate other confounding factors, including differences in physical activity and dietary habits of patients, which may have significantly impacted the results [46–53]. As reported in Table 2, another
Scientifica
3
Table 1: Observational studies investigating the relationship between vitamin D and carotid intima-media thickness.
Author
Year
Briese et al. 2006 [41]
Study design (sample size)
Country (ethnicity) Age
Correlation (lower range of 25(OH)D)
Findings
Case-control (40 ESRD patients and 40 matched healthy controls)
Germany (Caucasian) Mean 23.6 years
No (linear correlation)
There was no difference in CCA-IMT between the two groups. This study failed to correlate 25(OH)D and cIMT.
Italy (Caucasian) 50–65 years
Yes (
